chore(markdown): add message to copy to markdown

[shayna-ch]

adds ##Message section to the copy to markdown button, based off how seer includes it in the xml (<message>\n{message}\n</message>)

Example:

##Message 

Unrecognized Slack API message. Api Message: The request to the Slack API failed.

[shayna-ch]

bugbot run

[cursor]

✅ Bugbot reviewed your changes and found no new issues!

Comment @cursor review or bugbot run to trigger another review on this PR

^{Reviewed by Cursor Bugbot for commit ad4fce1. Configure here.}

[sentry]

Bug: The case-sensitive check group.title.includes(message) may fail to detect when the message is already in the title if their casing differs, leading to redundant output.
_{Severity: LOW}

Suggested Fix

Perform a case-insensitive comparison to correctly detect if the message is already in the title. Convert both group.title and message to the same case before the check, for example: group.title.toLowerCase().includes(message.toLowerCase()).

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent. Verify if this is a real issue. If it is, propose a fix; if not, explain why it's
not valid.

Location: static/app/views/issueDetails/hooks/useCopyIssueDetails.tsx#L278

Potential issue: The code uses a case-sensitive `String.prototype.includes()` to check
if an event's `message` is already contained within the `group.title` before adding it
to a generated markdown string. This is intended to prevent duplicating information.
However, if the `message` and its representation in the `title` differ only by case
(e.g., `title` is 'Error: Something failed' and `message` is 'something failed'), the
check will incorrectly return false. This results in the message being redundantly added
to the markdown output.

_{Did we get this right? 👍 / 👎 to inform future reviews.}

[shayna-ch]

doing it this way to match Seer